The present invention relates to color cathode-ray-tube (CRT) and, more particularly, a tension mask frame assembly having improved microphonics and thermal expansion behavior.
A conventional shadow mask type color CRT generally comprises an electron gun for forming and directing three electron beams to a screen of the CRT. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color-emitting phosphors. The shadow mask is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. In a majority of CRTs, the shadow mask is a domed thin sheet of metal capable of self-maintaining its configuration with the inner surface of the CRT faceplate and is supported by a mask frame. The mask acts as a parallax barrier that shadows the screen and permits the transmitted portions of the electron beams to excite phosphor elements of the correct emissive color on the CRT screen. Localized heating causes a doming-type deformation, which moves the mask apertures in relation to the fixed phosphor stripes thereby distorting the paths of the electron beams passing through the apertures between the strands, effecting misregister with the phosphor elements. Another type of mask commonly used in CRTs is referred to as a strand tension mask comprising a plurality of spaced apart thin parallel strands attached to a rigid mask frame. Such thin strands are basically non-self-supporting so they must be held in high tension so that tension is not lost when the mask expands thermally during operation. The tension on the strands ensures that the apertures formed between the strands remain in alignment with the phosphor elements on the screen. In these tension masks, even though localized thermal expansion of the strands is compensated by the tensioning of the strands, thermal expansion during tube operation can still cause the mask strands to move in relation to the fixed phosphor stripes thereby distorting the paths of the electron beams passing through the apertures between the strands, effecting misregister with the phosphor elements with resulting picture distortion.
Strand tension masks also have an inherent susceptibility to external vibration. Under tension, the strands tend to vibrate independently at a fundamental natural frequency. External influences such as the impact of the electron beam scan rates, mechanical shock, and vibration induced by a nearby loudspeaker or other sources of noises can stimulate large amplitude modes which can actively distort picture quality. Strand vibration can be damped by frictionally contacting each of the strands with a cross-wire attached to the mask frame. However, relying on cross-wires to provide positive and uniform contact on the strands is difficult to attain particularly when the associated strand mask is flat rather than curved.
Because of the negative effects of external vibrations, thermal expansion, and increased bulk and weight of the frame necessary for bearing the tensional strength of the strands, mask structures formed of light weight structures with low coefficients of thermal expansion are desirable. Thus, high cost iron-nickel alloy such as INVAR(copyright) is preferred over the low cost, low carbon alloy steel, since iron-nickel alloy materials have relatively low coefficient of thermal expansions (CTEs) as compared to low carbon alloy steels. Although such a structure is attractive from a performance standpoint, system costs are prohibitive from a manufacturing standpoint.
Hence, a need exists for a tension mask structure that overcomes the drawbacks of the prior art structure in maintaining a relatively precise spacing of the mask strands during manufacturing and tube operation.
The present invention provides a mask frame assembly having a plurality of spaced apart parallel strands. Each of the metal strands are attached at their ends to a strand termination insert having a lower coefficient of thermal expansion than the strands. The strand termination inserts are supported within insert receiving brackets located at two opposing sides of the mask frame. The mask also includes a plurality of cross-wires oriented substantially perpendicular to the strands. The cross-wires are attached to the strands and have similar coefficient of thermal expansion as the strand termination inserts.